There are known various types of flow meter which utilize a member which vibrates or undulates as fluid flows therepast. Fluid flow rates can be determined from the frequency of vibration or rate of undulation, which is generally proportional to the rate of fluid flow.
An early form of fluid meter having an undulating spring confined in a fluid flow chamber and a mechanical counter for indicating the amount of fluid flowing therethrough is described in U.S. Pat. No 398,013 (Welker). A more recent travelling wave flow meter, utilizing an undulating membrane formed of piezoelectric material which generates electrical signals as a function of the rate of undulation is described in U.S. Pat. Nos. 4,358,954 and 4,409,830 (both to de Jong).
The flow meter disclosed in the above-referenced U.S. Patents to de Jong is, however, limited in use to measurement of noncompressible fluids, such as liquids. If used for compressible fluids, such as gases, under a relatively high flow rate, the membrane would be liable to give rise to stretching thereof across a fluid flow path with which it is associated, so as to block fluid flow therealong.